Silage is made by harvesting crops of perennial legumes and grasses such as, for example, alfalfa, trefoil, clovers, orchard grass, annuals such as sudan grass, sorghum, timothy, bluestem and corn. The harvest takes place at the ideal maturity stage which, in the case of legume silage is full bud, in the case of grass silage the boot stage, in the case of corn silage, full dent, and in the case of sorghum sudan silage when fully headed. The proper moisture content for ensiling is approximately 65-75% by weight for corn silage, 50-60% by weight for legume/grass silage for large silos using a finer chop, 60-70% by weight for smaller silos and bunker/trench silos.
The harvested crop is processed through a chopping machine usually reducing the entire cut plant to pieces approximately 1/2 inch in length or less but more than 1/4 inch. These pieces are then placed in a silo or storage container. The most common container on the farm is the tall concrete structure silo. Some versions of the silo are glass-lined.
When forage crops are placed in the silo, sugars from these plants are converted by yeast organisms to carbon dioxide and water, along with heat production. Due to the limited oxygen supply in the silo, some of the sugars are converted to ethyl alcohol which in turn is oxidized to acetic acid due to the presence of acetic acid producing bacteria.
The production of acetic acid continues until the concentration is so high that the bacteria themselves are affected. The numbers of bacteria decrease and their activity in producing acetic acid stops.
Coincidental with the production of acetic acid by one or more types of bacteria, other types of bacteria produce lactic acid from the plant sugars.
The entire fermentation procedure generally takes about twenty days. If the fermentation has gone properly, all of the oxygen will have been used up in the silo or storage container. Furthermore, the contents of the silo will contain relatively high levels of acetic and lactic acids. This combination of materials and conditions will successfully prevent the growth of mold because mold growth is inhibited by a highly-acid environment and is generally stopped in an atmosphere devoid of oxygen. Molds are aerobic organisms.
If the fermentation procedure has not gone properly, there will be an insufficient amount of lactic acid present, allowing for the production of butyric acid. Furthermore, the lack of lactic acid allows for the reduction of protein and other nutrients in the silage, thereby reducing the nutritive value of the feed.